In a symmetrical Cassegrain both mirrors are aligned about the optical axis, and the primary mirror usually contains a hole in the centre thus permitting the light to reach an eyepiece, a camera, or a light detector. Alternatively, as in many radio telescopes, the final focus may be in front of the primary. In an asymmetrical Cassegrain, the mirror(s) may be tilted to avoid obscuration of the primary or the need for a hole in the primary mirror (or both).

The classic Cassegrain configuration uses a parabolic reflector as the primary while the secondary mirror is hyperbolic.[1] Modern variants often have a hyperbolic primary for increased performance( for example, the Ritchey–Chrétien design), or the primary and/or secondary are spherical or elliptical for ease of manufacturing.

The Cassegrain reflector is named after a published reflecting telescope design that appeared in the April 25, 1672 Journal des sçavans which has been attributed to Laurent Cassegrain.[2] Similar designs using convex secondaries have been found in the Bonaventura Cavalieri's 1632 writings describing burning mirrors[3][4] and Marin Mersenne's 1636 writings describing telescope designs.[5]James Gregory's 1662 attempts to create a reflecting telescope included a Cassegrain configuration, judging by a convex secondary mirror found among his experiments.[6]

Cassegrain designs

Light path in a Cassegrain reflector telescope

The "Classic" Cassegrain

The "Classic" Cassegrain has a parabolic primary mirror, and a hyperbolic secondary mirror that reflects the light back down through a hole in the primary. Folding the optics makes this a compact design. On smaller telescopes, and camera lenses, the secondary is often mounted on an optically flat, optically clear glass plate that closes the telescope tube. This support eliminates the "star-shaped" diffraction effects caused by a straight-vaned support spider. The closed tube stays clean, and the primary is protected, at the cost of some loss of light-gathering power.

It makes use of the special properties of parabolic and hyperbolic reflectors. A concave parabolic reflector will reflect all incoming light rays parallel to its axis of symmetry to a single point, the focus. A convex hyperbolic reflector has two foci and will reflect all light rays directed at one of its two foci towards its other focus. The mirrors in this type of telescope are designed and positioned so that they share one focus and so that the second focus of the hyperbolic mirror will be at the same point at which the image is to be observed, usually just outside the eyepiece. The parabolic mirror reflects parallel light rays entering the telescope to its focus, which is also the focus of the hyperbolic mirror. The hyperbolic mirror then reflects those light rays to its other focus, where the image is observed.

The radii of curvature of the primary and secondary mirrors, respectively, in the classic configuration are

Ritchey-Chrétien

The Ritchey-Chrétien is a specialized Cassegrain reflector which has two hyperbolic mirrors (instead of a parabolic primary). It is free of Henri Chrétien in the early 1910s.

Dall-Kirkham

The Dall-Kirkham Cassegrain telescope's design was created by Horace Dall in 1928 and took on the name in an article published in Scientific American in 1930 following discussion between amateur astronomer Allan Kirkham and Albert G. Ingalls, the magazine editor at the time. It uses a concave elliptical primary mirror and a convex spherical secondary. While this system is easier to grind than a classic Cassegrain or Ritchey-Chretien system, it does not correct for off-axis coma and field curvature so the image degrades quickly off-axis. Because this is less noticeable at longer focal ratios, Dall-Kirkhams are seldom faster than f/15.

Off-axis configurations

An unusual variant of the Cassegrain is the Schiefspiegler telescope ("skewed" or "oblique reflector", also known as "kutter telescope" after its inventor Anton Kutter[7]) which uses tilted mirrors to avoid the secondary mirror casting a shadow on the primary. However, while eliminating diffraction patterns this leads to several other aberrations that must be corrected.

Several different off-axis configurations are used for radio antennas.[8]

Another off-axis, unobstructed design and variant of the cassegrain is the 'YOLO' reflector invented by Arthur Leonard. This design uses a spherical or parabolic primary and a mechanically warped spherical secondary to correct for off-axis induced astigmatism. When set up correctly the yolo can give uncompromising unobstructed views of planetary objects and non-wide field targets, with no lack of contrast or image quality caused by spherical aberration. The lack of obstruction also eliminates the diffraction associated with cassegrain and newtonian reflector astrophotography.

Catadioptric Cassegrains

Schmidt-Cassegrain

The Schmidt-Cassegrain was developed from the wide-field Schmidt camera, although the Cassegrain configuration gives it a much narrower field of view. The first optical element is a Schmidt corrector plate. The plate is figured by placing a vacuum on one side, and grinding the exact correction required to correct the spherical aberration caused by the primary mirror. Schmidt-Cassegrains are popular with amateur astronomers. An early Schmidt-Cassegrain camera was patented in 1946 by artist/architect/physicist Roger Hayward,[9] with the film holder placed outside the telescope.

Argunov-Cassegrain

In the Argunov-Cassegrain telescope all optics are spherical, and the classical Cassegrain secondary mirror is replaced by a sub-aperture corrector consisting of three air spaced lens elements. The element farthest from the primary mirror is a Mangin mirror, in which the element acts as a second surface mirror, having a reflective coating applied to the surface facing the sky.

Klevtsov-Cassegrain

The Klevtsov-Cassegrain, like the Argunov-Cassegrain, uses a sub-aperture corrector. It consisting of a small meniscus lens and Mangin mirror as its "secondary mirror".[10]

Cassegrain radio antennas

A Cassegrain radio antenna - the 70 meter dish at JPL's Goldstone antenna complex. In this case the final focus is in front of the primary, at the top of the pedestal protruding from the mirror.

Cassegrain designs are also utilized in satellite telecommunications earth station antennas and radio telescopes, ranging in size from 2.4 metres to 70 metres. The centrally located sub-reflector serves to focus radio frequency signals in a similar fashion to optical telescopes.

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